You could use a proxy item that is initialised to be false on system start.
Once your rule triggers determines that you have a flooding situation, it can check your proxy item and only send an email if the state is false; after sending the email it would change the state to true;
finally one the value sinks to safe levels (to avoid oscillation this value needs to be lower than the level that makes the rule send an email; you may need to play with the gap of those thresholds), your rule can set the state of the proxy item to false again and you are right where you started with only one email sent.
